The present invention relates to the use of non-pyruvylated xanthan in oil field applications utilizing high density calcium-based brines.
Brines are used in oil field applications such as in drilling fluids and completion fluids. For instance, during drilling for oil, a drilling fluid is circulated to act as a lubricant and coolant for the drill bit. The drilling fluid also serves to suspend the cuttings and bring them to the surface. In general, a completion fluid is any fluid placed across a production zone prior to or right after perforating the well to bring on production. These fluids are also used to perform workover operations when attempting to improve productivity of a well. The primary objectives of a completion or workover fluid are to provide sufficient hydrostatic pressure to control abnormal subsurface pressures, eliminate the influx of formation fluids, maintain a stable wellbore, and ultimately, minimize formation damage to assure maximum production potential. These fluids also provide a media to remove debris from the wellbore including proppants, sand, metal cuttings and formation solids. Ideally, the fluid should contain a minimum amount of suspended solids which could plug pore throats of the payzone and interrupt production. Brines provide a preferred fluid since they provide a solids free media, yet provide a range of densities required to control formation pressures. Typically, clear brines ranging in density from 8.4 lb/gal to 19.0 lb/gal are used for completion and workover applications.
Brines useful as drilling and completion fluids are often composed of CaCl2 and are thickened prior to use. Important properties of such brines are viscosity and thermal stability. In order to provide the desired properties, xanthan gum is often added to viscosity the brine solution. Xanthan gum produces pseudoplastic rheology which means the viscosity is low under high shear conditions like pumping through the drill bit, but provides high viscosity under low shear conditions encountered in the annular region, thus transporting the cuttings from the wellbore to the surface.
In many cases, these brines are viscosified to:
Improve carrying capacity of the fluid to assure maximum solids transport from the well to the surface.
Provide suspension properties to enable the addition of sized particulates used to bridge pore throats and control fluid leak-off into permeable sections of the well.
Increase low shear rate viscosity to minimize radial flow and depth of fluid or filtrate invasion from the wellbore into a permeable formation.
Brines help minimize the hydration and swelling of smectite clays often encountered during the drilling process. These clays may be present in oil bearing formations, and if allowed to swell, will impede the flow of oil or gas from the reservoir during completion operations. Whereas traditional xanthan has been used to viscosify saline, mono-valent brines, the presence of high concentrations of divalent cations, such as calcium and magnesium, impede polymer hydration, making it difficult to viscosify calcium chloride brines.
It was discovered that pyruvate-free xanthan provides a means to viscosify a wide range of calcium chloride brines having densities from 9.0 lb/gal (1.08 g/cm3) to 11.6 lb/gal (1.39 g/cm3). Once viscosified, the calcium chloride brine provides a high density drilling fluid system that improves carrying capacity of drilled cuttings, suspension properties, and shale inhibition. These features help minimize problems often associated with inadequate hole cleaning, including stuck pipe, lost circulation, settling of solids in the wellbore and lack of directional control (in horizontal wells). Other benefits include the ability to suspend commercial solids such as calcium carbonate to increase the density of the fluid and provide sized particles to bridge fractures and/or pore throats in an effort to minimize fluid losses to the formation. Finally, calcium chloride formulated systems provide a media to minimize the swelling of in-situ clays which can lead to unstable wellbore conditions, or adversely affect permeability of a producing formation.
The present invention is therefore directed to a drilling, drill in, or completion fluid comprising a pyruvate-free xanthan, which may be an acetate-free pyruvate-free xanthan, and a high density calcium-based brine. Preferably the pyruvate-free xanthan is a genetically prepared pyruvate-free xanthan. The calcium-based brine is preferably a calcium chloride-based brine.
The present invention is further directed to a process for recovering crude oil from an oil-bearing subterranean formation by drilling the formation and using a drilling fluid comprising a pyruvate-free xanthan and a calcium-based brine.
The present invention is also directed to a process for recovering crude oil from an oil-bearing subterranean formation by using a completion fluid comprising a pyruvate-free xanthan and a calcium-based brine.